_utils: don't crash trying to flatten() strings.

Fixes #614.

amaranth/_utils.py

@@ -15,10 +15,10 @@ __all__ = ["flatten", "union" , "log2_int", "bits_for", "memoize", "final", "dep
 
 def flatten(i):
     for e in i:
-        if isinstance(e, Iterable):
-            yield from flatten(e)
-        else:
+        if isinstance(e, str) or not isinstance(e, Iterable):
             yield e
+        else:
+            yield from flatten(e)
 
 
 def union(i, start=None):

tests/test_hdl_ast.py

@@ -740,6 +740,11 @@ class CatTestCase(FHDLTestCase):
     def test_cast(self):
         c = Cat(1, 0)
         self.assertEqual(repr(c), "(cat (const 1'd1) (const 1'd0))")
+    
+    def test_str_wrong(self):
+        with self.assertRaisesRegex(TypeError,
+                r"^Object 'foo' cannot be converted to an Amaranth value$"):
+            Cat("foo")
 
 
 class ReplTestCase(FHDLTestCase):